Many synths have a feature known as a Low Frequency Oscillator or LFO. The name is confusing, because usually, an oscillator refers something that makes a sound, like a square wave.

In this case the low frequency oscillator does not generate a sound, rather it is a control that is used to dynamically tweak or modulate an existing sound to achieve different effects, making the sound more interesting.

It is called ‘low frequency’ because the frequency is usually at or just below the threshold of human hearing, usually less than 50hz, and often set to 20hz or 10hz.

Lets take a look at a few examples:

Imagine playing a note on your keyboard and turning the volume control knob rapidly back and forth with your fingers 10 times a second. By doing this, you are creating a tremolo effect. A 10hz LFO patched to the volume control would allow you to do the same thing without using your fingers.

Now imagine doing the same thing, but instead of turning the volume knob, you move the pitch bend wheel back and forth 10 times per second. This would create a vibrato effect. A 10hz LFO patched to the pitch bend control would do the same thing.

In fact, the LFO can be connected to a whole variety of controls or sound parameters to achieve different effects. The range of parameters you can control of course depends on the capabilities of your synth keyboard.

You can adjust the level (amplitude) of the LFO to determine whether you want a slight or a strong effect and you can raise or lower the frequency for faster or slower ‘throbbing’.

Digital Audio Workstation software and VSTs are usually extremely flexible in what you can control. In fact, you can use multiple LFOs running at different frequencies to control several parameters in parallel.